Aromatic polycarbonates belong to the group of the industrial thermoplastics. They are distinguished by the fact that they combine the technologically important properties of transparency, dimensional stability under heat and toughness.
In order to obtain high molecular weight polycarbonates by the interfacial process, the alkali salts of bisphenols are reacted with phosgene in a two-phase mixture. The molecular weight may be controlled by the amount of monophenols such as, for example, phenol or tert.-butylphenol. These reactions yield almost exclusively linear polymers. This may be demonstrated by end-group analysis. By the specific use of so-called branching agents, generally polyhydroxylated compounds, branched polycarbonates are also obtained in these reactions.
Regarding the preparation of polycarbonates by the interfacial process, reference is made by way of example to H. Schnell, Chemistry and Physics of Polycarbonates, Polymer Reviews, Vol. 9, Interscience Publishers, New York 1964 p. 33 ff and to Polymer Reviews, Vol. 10, “Condensation Polymers by Interfacial and Solution Methods”, Paul W. Morgan, Interscience Publishers, New York 1965, Chap. VIII, p. 325.
For the preparation of polycarbonates by the melt transesterification process, the bisphenols are reacted in the melt with diaryl carbonates, mostly diphenyl carbonate, in the presence of catalysts, such as alkali salts, ammonium or phosphonium compounds.
The melt transesterification process is described, for example, in Encyclopedia of Polymer Science, Vol. 10 (1969), Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964) and also in DE-C 10 31 512.
However, the known polycarbonates and copolycarbonates are unsatisfactory owing to their high melt viscosities or have the disadvantage that they may be limited in use to the production of molded articles having a large surface area, such as, for example, motor vehicle windscreens, or may not be optimally suitable.